The present invention relates to luminaries which are used with linear lamp sources beneath relatively low ceilings such as found in offices, schools, and shops. Indirect light fixtures are commonly provided when the use of direct types of lighting fixtures would introduce the element of unwanted glare and primary light source reflection onto such surfaces as the screens of visual display terminals, reading material, and outwardly displayed merchandise. Light fixtures for such use take many different forms and shapes which are determined by the aesthetic and performance criteria that are required. In a broad sense all or most of such indirect fixtures embody a single upwardly facing reflector assembly housed within a separate or integral shroud or enclosure which is then supported vertically below the ceiling surface of the room cavity which is to be illuminated.
It is known from Lambert's law that the illumination on a surface is proportional to the cosine of the angle of the incidence of the light ray, and that diffuse surfaces such as a white ceiling diffuse or scatter light incident from any angle and reflect it throughout a complete hemisphere in a generally cosine pattern. Further, it is known from the inverse square law with its cosine correction, E=(I/d.sup.2)cos.theta., defining the magnitude of illumination E at a point on a plane, that equality of the illumination at all points on the plane theoretically requires that a luminaire provide thirteen times more candlepower at a point approximately 2.15 times mounting height away on the plane to be illuminated than is provided at nadir. Despite this understanding of essential combined requirements for controlled, uniform, indirect illumination, attempts to design luminaries to take full advantage of the aforementioned principles have not been entirely successful.
A primary reason for the lack of success has been the inefficient shape, size, and composition of current reflector designs which are limited to the extent that the luminaries overall visual appearance often dictates the reflectors design criteria. It is generally accepted that an indirect luminaire should not exceed a maximum to minimum luminance ratio of more than 10 to 1, nor exceed a maximum footlambert level of 500 when measured above the fixture on the secondary ceiling surface. Failure to adhere to the aforementioned requirements results in uneven illumination as well as objectionable glare being produced.
Existing attempts to address the above mentioned performance criteria have used a greater quantity of smaller, lower brightness luminaries with upwardly exposed open lamp sources. This solution generally results in adequate uniformity and brightness levels being achieved at the expense of higher initial cost and increased energy consumption. Another method used has been to employ a luminaire with a lens or diffuser to shield and diffuse the lamp source from direct exposure to the ceiling surface. This solution decreases the lamp efficiency by a minimum of 8% to 10% thereby increasing the number of luminaries required for even illumination as well as increasing the initial cost of the luminaire. The lensed solution further contributes to the complexity and time required to maintain and clean the luminaire. Another method uses a lesser quantity of larger, higher brightness luminaries which typically utilize an HID (high intensity discharge) lamp source mounted well below the secondary surface which is to be illuminated. Negative features of the HID system include high initial cost, as well as an excessive mounting distance between the luminaire and the secondary surface, such distance being required to remain within acceptable brightness and uniformity criteria.
As discussed above, a common problem encountered with the use of indirect luminaires of this general type, i.e., the open or lensed lamp source being directly exposed to the secondary reflective surface, is that an inordinately high number of luminaires and/or lamps are required to provide a specified level of even illumination on the ceiling or secondary surface. As further discussed above, the present fixtures should be suspended a minimum of 9 inches or more from the ceiling surface in order to remain within the acceptable brightness criteria. The present trend towards energy conservation has established the need for an efficient, cost effective indirect luminaire that can achieve the required levels of uniform illumination by utilizing a significantly smaller number of luminaires and/or lamps than would otherwise be required by use of conventional indirect luminaires.
A need has thus arisen for an indirect light fixture having increased efficiency with low cost of manufacture and operation.